Heat shrinkable plastics tubing is used for labelling electric cables and the like, with each separately labelled section, or labelling zone, of tubing being cut or perforated to allow subsequent tearing, and then threaded over the corresponding cable before being heat shrunk to secure it in place on the cable. Because of the difficulty of printing direct onto the, normally flattened, plastics tubing, labels are generally mounted independently onto a carrier which is then fed through a printer. This of course is costly, involving many separate operations including pre-cutting the plastics tubing into lengths and fixing them to a roll of carrier material, and is difficult to automate. Alternatively pre-marking of the printable media is necessary. In our UK patent GB 2,387,142 we describe a new assembly, apparatus and method to enable flattened plastics tubing to be printed directly with a sequence of labels along its length.
That printing assembly comprises at least one supply of printable medium and a supply of control medium arranged on a common support such that both media may be passed together, in use, along adjacent paths at the same speed through printing apparatus, the supply of control medium having register control marks along its length, the register control marks being detectable by the printing apparatus for controlling the register of printing operations on the supply of printable medium performed by the printing apparatus as the printable and control media are passed through the printing apparatus.
Different sizes of printable media may be accommodated by this assembly using a guide comb within the printing apparatus that has a number of stepped channels through which the printable and control media run. This guide comb arrangement serves to maintain the respective lateral positions of the printable media and control medium but has been found not to be wholly reliable, especially in the context of the heat shrinkable plastics tubing
It is thus one of the objects of the present invention to provide printing apparatus for printing heat shrinkable plastics tubing or similar printable media with enhanced reliability, substantially ensuring that the printable media and control media, if any, are maintained in the appropriate laterally spaced relationship during the printing.
More importantly still, a primary objective of the present invention is to provide an apparatus and method to enable flattened plastics tubing or other printable medium to be printed direct with a sequence of labels along its length on both sides of the printable medium in a cost and space efficient manner. This most preferably is done in a way that ensures register between the label content printed on one side of the medium with the label content printed on the other side of the medium. Prior attempts to provide double-sided printing have entailed use of two printing devices in tandem but this is prohibitively costly for most and no feasible alternative has previously been proposed.
Additionally, after extensive use of the control medium-registered printing apparatus and further development we have unexpectedly found a further, simpler, substantially effective and cost effective solution to the problem of controlling the print registration that avoids the need for provision of a control medium or any register marks. Accordingly, a yet further objective of the present invention is to enable flattened plastics tubing or other printable media to be printed direct with a sequence of labels along its length without need for providing register marks or other means to identify on the printable media the location for the start or end of each label.